The present invention relates in general to retrofit fluorescent lamp dimming and more specifically to a closed-loop electronic dimming control and to a control method for a fluorescent lamp dimming system.
Historically, the realization of a dimmable fluorescent lighting system which is inexpensive and convenient to install and use has been very elusive. The vast majority of fluorescent lighting units in use are, consequently, of the nondimming type, most using filament-heated lamps and the rapid-start type of ballast. However, these units tend to be application inefficient since they often produce more light and consume more energy than required in a particular situation.
Fluorescent lamp dimmers are known which are capable of dimming the light output of fluorescent lamps connected in conventional ballasts by conditioning the AC power supplied to the ballast, and which are retrofitable. In one exemplary prior art device, such as is shown in U.S. Pat. No. 4,350,935 to Spira et al., a portion or portions are removed from each half-cycle of the AC input waveform (by opening a switch in series with the lamp ballast and power supply) resulting in periods of zero energy transfer to the lamps. The overall reduction in energy transfer results in a lowered light output from the lamps.
An improved fluorescent lamp dimming system of the prior art conditions the AC power supplied to the lighting system by chopping the input waveform in order to provide a variable duty cycle, low frequency input component which gives a variable light output, and to provide a high frequency component (at least 10 times greater than line frequency) for maintaining filament heating at any amount of dimming without adding substantially to the light output. This improved system is described in U.S. patent application Ser. No. 645,593 of Alley et al., filed Aug. 30, 1984, which is of common assignment and which is hereby incorporated by reference.
Fluctuations in line voltage supplied to a lighting system can cause undesirable changes in total light output. It would be advantageous to stabilize light output over a wide range of operating conditions, including sizes of the load connected to the dimming system.
A dimming control product must be loss-less or have very low losses in order to maximize the energy savings of dimming. Not only should the dimming system maintain a correct average light level, but it should also not be subject to lamp flicker from DC offset voltages.
Accordingly, it is a principal object of the present invention to provide an electronic control and a control method for dimming fluorescent lamps.
It is another object of the invention to provide closed-loop feedback in a fluorescent lamp energy management/dimming system.
It is a further object of the invention to provide a fluorescent lamp dimming system which is insensitive to voltage fluctuations and DC offsets.
It is yet another object of the invention to provide a dimming system of low energy consumption and low cost, which is also retrofitable.